The present invention relates to an electrode membrane having selectivity for lithium ions.
It is eagerly desired to develop reliable lithium ion-sensitive electrode membranes used, for example, for the determination of lithium ions in blood serum in the clinical inspection of patients suffering from manic depressive pyschosis.
Various types of polymeric membranes have been hitherto proposed and studies to provide a membrane electrode having selectivity for lithium ions. The polymeric membrane of this kind has a basic structure formed of a polymeric composition composed of a film-forming polymeric resin, a plasticizer, an ion-sensitive substance called an ion carrier and, optionally, an alkali metal salt of a hydrophobic anion. For example, a lithium ion-selective polymeric electrode membrane can be prepared from a polymeric composition comprising a polyvinyl chloride resin, a plasticizer such as 2-nitrophenyl octyl ether, an ion carrier such as N,N'-diheptyl-N,N'-5,5-tetramethyl-3,7-dioxsanonane diamide and, according to need, an alkali metal salt of a hydrophobic anion [see, for example, E. Metzger et al., Helv. Chim. Acta, volume 69, page 1821 (1986) and K. Kimura et al., J. Chem. Soc. Perkin Trans., II, 1986, page 1945].
None of these lithium ion-selective polymeric membranes, however, is quite satisfactory in respect of the selectivity for lithium ions relative to other alkali and alkaline earth metal ions.